Head pressure is calculated and represented in terms of feet. In order to calculate the total head pressure of your return pump, you'll need to know the difference in elevation between your pump and return outlet. You will also need to know the total number and type of plumbing fittings used as well as the total distance of pipe/tubing used.
The effect of gravity on head pressure is very simple. Every vertical foot of distance the return pump moves water equates to 1 foot of head pressure. The effects of friction on head pressure are more difficult to calculate. Roughly every 10 feet of pipe (horizontal & vertical) through which water is traveling adds 1 foot of head pressure. Additionally, every 90-degree bend adds 1 foot of head pressure.
Pipe diameter is also an extremely important factor when calculating head pressure. As a general rule of thumb, using a smaller diameter pipe than the return pump output will drastically increase head pressure. For minimum head pressure, using the largest diameter pipe possible is best. Avoiding things like flow meters, reactors, and chillers will also help to reduce head pressure.
If you're unsure what Vectra is best for your aquarium, it would be best to get a general idea of your head pressure first. From there, calculating a rough gallons per hour (GPH) requirement for your aquarium and equipment is important. Based on that information and the chart below, you can then determine which Vectra is best for your aquarium.